This invention relates to apparatus for manufacturing articles by thermoforming a heated sheet of thermoplastic material, the pre-heated sheet being intermittently fed to a work station and there subjected to thermoforming and stamping due to the cooperation of a forming tool and a stamping tool movable in relation to each other.
In the mass production of articles by thermoforming by means of vacuum and/or compression, manufacture is generally in accordance with two known methods. In both cases a roll of sheet material is used as the starting material. The sheet material is fed from a suitable reeling means, through a pre-heating zone to give the material a suitable temperature for the forming process. The sheet material heated in this way is then introduced into a forming and cooling zone where the requisite forming tool and cooling means are provided. In certain cases the forming tool may consist of a simple vacuum-forming tool and in other cases it may consist of a combined tool so that forming is performed with the aid of compressed air from one part of the tool and possibly with the help of vacuum applied in the other part of the combined tool. In certain cases only compressed air forming is used.
After forming and cooling, the articles thus formed are separated from the sheet of material and are then stacked, counted and so on in suitable manner in equipment designed for each specific requirement.
The separating and stacking in conjunction with or after the forming process is carried out in principle using two different methods. In the case of relatively thick sheets of plastic the separating and stacking are often performed in a separate station. An example of this is the mass production of disposable drinking mugs. In such a case a large number of mugs is often formed in each forming operation. After cooling, the material is transported suspended freely to a separating and stacking station. The separating is often performed as a complete punch and die stamping operation which requires two opposed tools aligned with great precision. To avoid the forces required for the stamping operation becoming unreasonably great, stamping is usually performed so that in each stamping stroke, only a single transverse row of the formed mugs is stamped out simultaneously. This means, for instance, that if 64 disposable mugs have been formed in 8 transverse longitudinal rows in relation to the sheet of material, the stamping tool will only stamp out one transverse row of 8 mugs at each stroke. In this case, therefore, the stamping tool must perform 8 strokes for each stroke of the forming die. This means that the forming and stamping stations must be synchronised so that the stamping tool operates at the correct point with as little deviation as possible. This is achieved, for instance, by producing small protrusions on the sheet of material during the forming operation. These protrusions are preferably applied along the edges of the sheet material and are effected so that a gripping means can push or pull forward a suitable amount of the sheet material between successive strokes of the stamping tool.
The articles stamped out in accordance with this system are often stacked directly in association with the stamping operation, one of the stamping tools moving the freed articles on to a collecting means of suitable type.
The method described above for mass production of thermoformed articles is suitable for relatively thick sheets of material, particularly as in this case compressed air must be used, possibly combined with vacuum, in order to achieve an acceptable production rate.
However, the method described above is not suitable for the mass production of thermoformed articles from relatively thin sheets of material. The primary reason is that it is difficult to achieve the requisite control over the feeding of the sheet material between the forming and stamping stations to enable acceptable accuracy in the stamping operation. The same problem exists with thermoplastic material which becomes distorted after the forming operation. It is difficult in this case also to achieve a good stamping operation with sufficient accuracy.
A different method is often used for mass production from thin sheet material, according to which the sheet material is fed forward in a stretched state by means of a single feeding arrangement through pre-heating, forming/cooling and stamping stations.
Obviously in this method the stamping operation must be synchronised with the forming operation and be performed in the same number of strokes as the forming operation. This means that the stamping tool must be as large as the forming die. This, on the other hand, entails the drawback that the stamping tool is large and requires considerable stamping force. There are thus considerable disadvantages and in mass production, i.e. when, for instance, a large number of articles is produced in a single forming stroke, it also means that the stamping tool will be extremely expensive and complicated. In some cases the problem has been solved by, instead of a complete punch and die stamping operation, performing the stamping with a tool consisting of steel blades or strips which stamp through the sheet material against a flat, solid support. In this case the whole contour is not severed and instead the stamping strips are arranged so that the material is not stamped all the way through at some points. In this way the products will remain suspended in the sheet material after stamping. However, they can be pushed out of the sheet material for stacking by suitble means in a separate stacking station arranged after the stamping station and operating synchronously therewith, after which the remains of the sheet material is coiled up.
The method described above has the advantage that it is particularly suitable for thin sheets of material or sheets which have a tendency to shrink during the forming process. The disadvantage, however, is that considerable stamping force is required here too since in this case the stamping tool is as large as the forming die. Another drawback is that it is not always possible to achieve the precision necessary in the stamping for some articles.
If great precision is required at the stamping stage, forming and stamping are performed in the same station, the forming die being built into the stamping tool. However, for stamping and forming by means of a combination of compressed air and vacuum, an extremely complicated tool is required which is expensive to manufacture. Since the pressure required is high the size of the tool is limited, which also limits its capacity. There are also problems in removing the articles after stamping. This is often done by pneumatic transport. This principle, therefore, is also complicated and is not suitable for the manufacture of thin-walled products as these cannot be transported or stacked pneumatically since the walls are too weak.
In this case of simultaneous vacuum-forming without compressed air and combined with stamping in the forming die, the tool becomes simpler and good precision in the stamping stage is also obtained, of course, but even in this case considerable stamping force is required and the tool is thus limited in its size. If a thin foil is to be formed to a thin-walled article, the same difficulties as mentioned above exist in dealing with the stamped-out article for stacking. This can be solved by not stamping completely through the plastic foil but, in a manner similar to that previously described with strip stamping, leaving a few points around the stamping contour so that the product is only partially stamped out. The article is then able to accompany the sheet of material out of the forming zone. The articles are then pushed out of the sheet material in suitable manner in conjunction with stacking. The method just described thus gives the advantage of good precision in the manufacture of thermoformed, thin-walled products from thin sheets of thermoplastic material. However, as mentioned, the drawback is that the number of articles formed simultaneously will be limited by the punching force prevailing and that the thin-walled, and thus possibly delicate, articles can only be separated from the sheet of material with a certain amount of difficulty without a risk of the articles being mechanically deformed. Furthermore, as is known, guide-strip stamping with only partial punching out results in small pieces of material being left at the points where the product is attached to the sheet material, which may be a drawback in many cases.
The object of the present invention is to provide apparatus suitable for mass production of theroformed, thin-walled, substantially cup-shaped products where the stamping operation must be carried out with precision and where the products are formed from a thin sheet of thermoplastic material.
A subsidiary object is to provide apparatus which require only negligible stamping force and thus enables the use of large tools, i.e. simultaneous stamping of a large number of articles.
A further subsidiary aim of the invention is to provide total stamping-out of the articles so that these need not be separated at a later stage, as well as a high-production rate since part of the cooling process should occur during and after stamping.
Finally, it is yet another subsidiary aim of the invention that the articles thus stamped out in conjunction with forming are easily accessible for transfer to a stacking and/or packing station.